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Valence Band Tuning of Electrocatalysts for the CO2 Reduction Reaction

Periodic Reporting for period 1 - CO2RR VALCAT (Valence Band Tuning of Electrocatalysts for the CO2 Reduction Reaction)

Reporting period: 2019-06-01 to 2021-05-31

The electrochemical reduction of CO2 enables carbon-neutral fuels and chemicals to be produced using intermittent renewable electricity. Commercial implementation of this technology would facilitate EU policy goals associated with the mitigation of global climate change by providing a means of utilizing CO2 as an industrial feedstock. Unfortunately, monometallic Cu is the only electrocatalyst that exhibits a significant Faradaic efficiency for hydrocarbon and alcohol formation and its voltage efficiency is not sufficient to make the process economically viable.1 This issue is compounded by the fact that electrocatalyst discovery efforts over the past 30 years have failed to identify alternative materials for catalyzing this reaction, let alone with superior activity. However, these studies have revealed that the reduction of a CO intermediate is the rate-determining step of the reaction. Thus, the unique ability of Cu to catalyze CO2 reduction to hydrocarbons and alcohols has been attributed to its moderate CO adsorption energy, which is unique among transition metals.2 The unique CO adsorption energy of Cu is thought to be the result of its unique d-band structure.3 Thus, if the d-band structure of another metal could be modified to resemble Cu then it might also exhibit the ability to catalyze CO2 reduction to hydrocarbons and alcohols. Furthermore, systematic electronic structure tuning provides a viable route toward the discovery of electrocatalysts with superior activity. Such a discovery could enable this process to be economically viable. Electronic modifications of this magnitude can be induced through intermetallic bonding between electronically dissimilar metals in what are known as intermetallic alloys.4 Intermetallic alloys have received considerable interest as advanced catalysts for thermally catalyzed reactions but have not received concomitant interest in the electrocatalysis community. However, several Cu-free intermetallic alloys have been shown to exhibit Cu-like electronic structures, chemical reactivities, and catalytic activities for a variety of thermally catalyzed reactions, such as methanol synthesis and steam reforming.5
The CO2RR VALCAT project sought to investigate the role of systematic valence electronic structure modifications on the CO reduction activity of Cu-free intermetallic alloys. The goals of the project were to:
1. Identify the electrocatalyst properties required to catalyze CO reduction. The unique ability of Cu to catalyze CO reduction is likely a result of its unique valence electronic structure. This hypothesis was explored by synthesizing Cu-free intermetallic alloys with nearly identical valence electronic structures and measuring their electrocatalytic activity for CO reduction.
2. Demonstrate the impact of systematic electronic structure modifications on the CO reduction activity of transition metals. The d-band portion of the valence band density of states of a transition metal can be systematically modified via the formation of a strong heteronuclear bonds with electronically dissimilar metals, such as those found in intermetallic alloys. We will investigate the extent to which surface reactivity can be tuned through systematic d-band modifications and will demonstrate the impact such modifications have on catalytic activity and selectivity.
3. Discover novel electrocatalysts for CO reduction with superior activity to Cu. The strong heteronuclear bonds characteristic of intermetallic alloys result in periodic structural order. Co-locating different metal sites results in the formation of catalytically active motifs that can enhance the stability of transition states though bidentate binding to metal sites with disparate chemical reactivity, as are present in many enzymes.


References
1. Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 14 (1985).
2. Peterson, A. A.; Nørskov, J. K. J. Phys. Chem. Lett. 3 (2012).
3. Nilsson, A.; Pettersson, L. G. M.; Hammer, B.; Bligaard, T.; Christensen, C. H.; Nørskov, J. K. Catal. Lett. 100 (2005).
4. Bligaard, T.; Nørskov, J. K. Electrochimica Acta 52 (2007).
5. Iwasa, I.; Masuda, S.; Ogawa, N.; Takezawa, N. Appl. Catal. A 125 (1995).
The project successfully investigated the impact of constituent identity and composition on the valence electronic structure of several Pd-based intermetallic alloys. Several different intermetallic phases exhibited Cu-like valence electronic structures and were demonstrated to also exhibit Cu-like CO binding energies, validating the notion that such electronic modifications may significantly alter the electrocatalytic activity of the constituents of the alloy. However, a systematic investigation of the impact of air exposure on the near-surface composition of these alloys revealed that significant structural changes occur instantaneously upon air exposure. This phenomenon impeded the accurate investigation of the electrocatalytic activity of these materials. To address this issue, a general experimental protocol was developed and validated that prevented such multi-metallic surfaces from undergoing changes in air by protecting them from oxidation using a sacrificial passivating overlayer that can be selectively removed in an electrochemical environment. A new type of electrochemical reactor for fundamental studies of CO reduction with reduced mass transfer limitations was designed and demonstrated to be capable of intrinsic activity measurements at rates roughly an order of magnitude higher than conventional lab scale reactors. However, when performing CO reduction over standard Cu electrocatalysts significant deactivation was observed, amounting to a 90% activity loss in 2 hrs. This phenomenon had not been widely acknowledged nor explained in the contemporary literature. However, careful post-reaction surface characterization revealed that deactivation occurs via the electrodeposition of trace amounts of carbophilic contaminants onto the electrocatalyst surface during CO reduction. These carbophilic contaminants then catalyzed coke formation, which rapidly deactivates the electrocatalyst by blocking active surface sites. Removal of these trace contaminants yielded vastly superior stability, with only 40% deactivation observed over 20 hrs. The CO reduction activity screening of promising intermetallic electrocatalysts is ongoing work.
The discovery of an experimental method to bridge the pressure gap between ultrahigh vacuum conditions, where the electronic structure measurements are performed, and ambient conditions, where the activity measurements are performed, will accelerate the discovery of highly active electrocatalysts for a variety of electrosynthesis reactions. Furthermore, the development of lab-scale electrochemical reactors with reduced mass transfer limitations will facilitate the development of electrosynthesis technologies by enabling electrocatalyst screening under conditions more closely resembling those found in commercial devices. The elucidation of electrocatalyst deactivation pathways enables these deleterious phenomenon to be suppressed or eliminated altogether, ensuring sustained device performance. Finally, establishing relationships between the electronic structure and electrocatalytic activity of materials will provide viable routes toward the development of highly activity catalysts, which will ultimately enable such processes to be economically viable.
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